There is an increasing interest for photovoltaic cells due to the improvement of their energy efficiency. Such photovoltaic cells are based on the use of a photovoltaic stack of active materials, which absorbs an optical energy, for example solar energy, and converts it into electric current coming from an electrochemical potential difference between the active layers. The photovoltaic stack can formed inside a bulk material (a sheet of crystalline or polycrystalline silicon) or thanks to materials deposited as thin layers onto a substrate. The potential difference may come from the doping of a material or the use of different materials. A photovoltaic cell also comprises electrodes on either sides of the photovoltaic stack, to collect the thus-generated electric current and to transport it laterally to the edges of the cell. A photovoltaic module is formed by connecting several photovoltaic cells in series and/or parallel.
The metal electrodes are generally opaque to the visible light. However, electrodes have been developed, which are both transparent and conductive and which can be placed on the surface exposed to the optical radiation of a photovoltaic stack. For the bulk material cells, the doped layer opposite the illumination source (hereinafter, this layer will be referred to as “the emitter”) can be conductive enough—for example if it is made of a doped bulk material—to transport the charge carriers laterally over distances of the order of the millimeter. Then, a metal mesh grid is used to transport the current over the remaining route to the edges of the cell. Often, for the thin layers, the doped active layer is not conductive enough to efficiently carry out this transportation, and another electrode material that is both transparent and conductive is necessary. The materials used for these transparent electrodes are in particular metal oxides (TCO, for Transparent Conductive Oxides) that are typically metal oxides doped with another element, such as fluorine-doped tin oxide (SnO2:F) or aluminium-doped zinc oxide (ZnO:Al).
For the photovoltaic cells obtained by stacking of thin layers, two types of structure exist. According to a first type of structure, of the “superstrate” type, a transparent substrate serves as a window for the photovoltaic cell. In this case, a transparent electrode is deposited as a thin layer of transparent conductive oxide (TCO) onto the transparent substrate (a glass sheet, for example). The photovoltaic stack and the back electrode are then deposited successively according to the known deposition methods. According to a second type of structure, of the “substrate” type, a metal electrode is deposited onto a substrate (not necessarily transparent), then the thin layers of photovoltaic materials are deposited above, and finally a transparent electrode is deposited above the photovoltaic stack. The thus-formed cell is then encapsulated.
In the photovoltaic cells based on a bulk photovoltaic material, this material serves as a substrate, a metal electrode is deposited onto the back face of the material, and the emitter on the front face (produced by thermal diffusion of a dopant or by ionic implantation) takes part in the lateral transportation of the carriers. A metal grid is deposited so as to complete the conductive circuit for the carriers up to the external electrodes. The HIT (Heterojunction with Intrinsic Thin Layer) cells constitute a particular case, because they use a bulk material but the doped layers are deposited from gas-phase precursors and are not conductive enough. Therefore, this type of cell requires a TCO layer for the lateral transportation of the carriers, as in the case of cells based on thin layers deposited onto a substrate. Moreover, the bulk silicon cells can also use a TCO layer as an antireflection layer.
To enter a photovoltaic cell, whatever the type of cell used, the light passes through a window that must fulfil two functions: it must be as much transparent as possible to allow a maximum flow of light to pass through, and it must be as much conductive as possible to minimize the ohmic losses when the photo-current is collected. However, the transparent electrodes have lower electric properties than the metal electrodes.
In order to increase the current collection by a transparent electrode, certain devices use a window that comprises a thin layer of a transparent conductive oxide (TCO), used either alone or in combination with a metal grid. The use of such a metal grid is, for example, described in the patent document CA1244120. In the cell areas that are close to the connection where the current will be possibly extracted, the surface area occupied by the fingers of the grid increases: the fingers are there closer to each other and/or wider. The surface area occupied by the fingers of the grid becomes more important, and thus a greater surface area of the cell is masked by the fingers, however the obtained increase of lateral conductivity is clearly advantageous for the cell efficiency.
For the thin-layer cells, it is common to use only a TCO layer without a metal grid. The commonly used TCO materials and the thicknesses thereof are, for example: SnO2:F (800 nm), ZnO:Al (600 nm) and ITO (200 nm). ZnO:Al is preferably used in applications in which the photovoltaic thin layer is deposited above the TCO layer, in conditions rich in hydrogen, because this TCO layer alone is resistant to reduction by the atomic hydrogen generated in the plasma during the deposition. However, the absorption in the layer of ZnO:Al increases at high wavelengths with an increasing doping rate, as described by Berginski et al. (SPIE Photonics 2006), so much that the constraints of low absorption and low resistivity are opposite to each other, at the level of the material properties themselves. This constraint is added to the evident constraint linked to the fact that a thicker layer has a higher lateral conduction but also a higher absorption. A compromise has thus to be found between these two properties in the structure of a cell or a photovoltaic module.
Particular structures have been developed to try to avoid these drawbacks. The U.S. Pat. No. 4,647,711 describes the inclusion of a current-collector metal grid in the TCO layer. The document WO 2008/005027 describes a photovoltaic cell comprising a conductive layer electrically connected to an electrode grid using trapezoid contact areas.
More particularly, for cells based on crystalline silicon wafers, the document JP2004214442 describes a photovoltaic cell comprising a collector metal grid deposited onto an ITO layer with a regular thickness but with an oxygen concentration rate varying between two values, according to whether the area of the ITO is located below the metal grid or not, in such a manner that the areas that are not below the grid have a lower light absorption coefficient than the areas located below the grid. In this case, the emitter is made of bulk and crystalline materials, and thus is conductive enough to contribute to the lateral conduction.
Another critical element in the manufacturing of photovoltaic modules is the interconnection between the cells. A single one cell provides, through its electrodes, a voltage close to the separation between the Fermi levels of the two doped materials in the cell (˜1 V or less), with a current that is proportional to its surface area (10-30 mA/cm2) and thus relatively high (e.g. 10 A for a cell of 20 cm in diameter). The electric power in such an embodiment is not sufficient in a practical point of view, and accordingly, in order to obtain a higher voltage, the cells are interconnected in series to produce a module. For monolithic cells based on crystalline silicon wafers, each cell comprises a complete wafer. The connections in series to form a photovoltaic module are then made by connecting together the conductors coming from each individual cell. For thin-layer photovoltaic modules—as those that use a layer of absorbent material made of amorphous silicon, microcrystalline silicon, either amorphous or microcrystalline silicon and germanium alloy, cadmium-based alloy, or copper, indium, gallium and sulphur alloy—the interconnections between cells are made through masking, deposition and/or etching steps.
Many publications describe optimized implementation methods using techniques such as lift-off, exposure or filling with different materials (WO2008074879, WO2008038553, WO2008016042, US2001037823), as well as laser etching steps (JP2002280580, U.S. Pat. No. 5,981,864, EPO422511). Other documents describe modules in which the cells are connected in parallel (U.S. Pat. No. 6,011,215, CA1227861, U.S. Pat. No. 4,652,693, U.S. Pat. No. 4,745,078).
The thin-layer photovoltaic cells and modules available on the market generally use a layer of high-quality Transparent Conductive Oxide, the thickness, transparency and electric conductivity of which are uniform over the surface area of a cell. However, the photovoltaic efficiency of these cells is not optimum. The interconnections and the distances between theses interconnections are optimized to take into account the fact that no current is generated within an interconnection. From this point of view, the distance between interconnections must be maximized. On the other hand, a great distance between interconnections causes a greater series resistance due to the layer of TCO. The use of a uniform layer of TCO thus leads to a single solution for the optimum distance between interconnections.